The development of high-pressure discharge lamps with metal halide fillings, has gone increasingly towards lamps with a low power input, more particularly lamps with a power input of less than 100 W. A great problem with this type of lamp is fast and reliable starting of a cold lamp, as well as restarting of the hot lamp, while maintaining a life of satisfactory length. Slow and difficult starting, which is due to unstable emanation of the arc from the electrodes, causes rapid blackening of the arc tube. The metal halides in the filling corrode the electrodes, especially in lamps of low power input in which the electrode shank has very small dimensions. Both the blackening of the arc tube and the corrosion of the electrodes result in the premature failure of the lamp.
Unstable arcing from the electrodes, i.e. power to transfer between electrode and the arc is caused primarily by the slow heating of the electrodes of this type generally have substantial heat loss. The German Patent Disclosure Document DE-OS No. 29 51 967 (to which British Pat. No. 2 043 331 corresponds) solves this problem for a double-ended miniature type metal halide discharge lamp by providing a self-heating electrode. The electrode comprises a current lead shank with a metal cap. A coil, about which a tungsten wire has been wrapped, is clamped on the metal cap. The construction of this electrode is very complex and is therefore very difficult to use in a miniaturized high-pressure discharge lamp of low power input.
The corrosion of the electrodes which begins especially at the electrode shank may be prevented, for example, by placing a small ceramic tube over the shank. Frequently, however, this causes problems when the ceramic material is sealed to the arc tube. The British Patent Specification GB-PS No. 1 242 173 describes a double-ended metal halide discharge lamp of 400 W wherein the electrodes have the form of a densely wound coil and are arranged at a right angle to the axis of the arc tube. This results in the ends of the arc tube being heated better, and reduction of the condensation of the metal salts in these regions. Corrosion of the electrodes is thus largely prevented. Turning the electrodes through 90.degree. in a double-ended arc tube--the axes of the electrode coils include a substantially right angle with the arc--causes, however, quicker sputtering of the electrode tip and is disadvantageous for high-pressure discharge lamps of low power input as will be shown below.